1. Field of the Invention
The present invention relates to a burst mode optical receiver, and more particularly, to a burst mode optical receiver which improves receiving sensitivity by automatically controlling gain of a pre-amplifier.
2. Description of the Related Art
FIG. 1 illustrates a passive optical network (PON) using a conventional burst mode receiver (not shown). In the PON, TDMA (time division multiple access) is used for multipoint access in order to reduce cost. In TDMA, each user packet is multiplexed in a time sequence as burst data at an optical line termination (OLT) 30 which receives optical packets from a plurality of optical network units (ONUs) 10a-10d via a passive optical splitter. Splitting of optical signals is caused by difference among distances between the OLT 30 and the ONUs 10a-10d. The conventional burst mode receiver in the OLCT 30 receives optical packets with different signal amplitudes, i.e., optical packets with burst characteristics due to splitting of the optical signals. As such, the conventional burst mode receiver must be able to handle these burst packets.
FIG. 2 is a block diagram of a conventional burst mode optical receiver in the form of a feed-forward scheme (refer to U.S. Pat. No. 5,475,342). Referring to FIG. 2, an output signal INPUT of a pre-amplifier (not shown) is used as inputs for a limiting amplifier 70, a top peak detector 50, and a bottom peak detector 60. A reference voltage Vref, which is set based on the top peak detector 50 and the bottom peak detector 60, is used as another input for the limiting amplifier 70. When the burst mode optical receiver receives a burst signal of small amplitude which meets the receiving sensitivity of the receiver, it is difficult for the top peak detector 50 and the bottom peak detector 60 to accurately detect a top peak voltage and a bottom peak voltage. As a result, the reference voltage Vref, an average value of the top peak voltage and the bottom peak voltage, cannot be accurately set, which increases the probability of pulse width distortion in a final output of the limiting amplifier 70.
FIG. 3 is a block diagram of a conventional burst mode optical receiver in the form of a feed-back scheme (refer to U.S. Pat. No. 6,055,279). In the conventional burst mode optical receiver of FIG. 3, outputs of a main pre-amplifier 100 and a tracking pre-amplifier 110 are respectively input to an OP-AMP 120, and an output of OP-AMP 120 is fed back to OP-AMP 120 via an automatic threshold controller (ATC) 130. The conventional burst mode optical receiver of FIG. 3 does not compensate for a burst signal of small amplitude which meets the receiving sensitivity of the receiver, which increases the probability of pulse width distortion when the burst signal of small amplitude is detected.